This invention was specifically developed for the surgical joining of bone bodies, such as the fusing of contiguous spinal vertebrae so as to stabilize and prevent relative motion often resulting from a degenerative disc condition. Although the immediate effort leading to this disclosure is directed toward the lumbar, thoracic and cervical spine (anterior or posterior in approach), the described vertebral implants for immediate fixation and staged stabilization leading to arthrodesis (bone fusion) of bone bodies may be used in a bone fracture or osteotomy to fuse together resulting bone bodies, and across one or more joints or articulations. Furthermore, the implants may be used in the lumbar, thoracic and cervical spine.
The use of fixation plates and screws to hold together disunited bone bodies has long been known to facilitate arthrodesis or bone-to-bone union, such as bone fusion, and healing of fractured bones. Typically, the separate bone bodies are formed when a single bone fractures, requiring bone reunion. Plates are secured across a fracture region with screws, joining together the bone bodies. The plates hold the bone bodies together in proximate relation, facilitating bone growth and fusion therebetween. In this manner, the bone bodies are supported in close proximity, or in direct contact which facilitates fusion therebetween. For cases where it is impossible to fixture together bone bodies internally of a patient's skin, external fixation is used. For external fixation, threaded pins are rigidly secured into each bone body. The pins, which extend outwardly of a patient's skin, are fixtured together with an external fixation device, placing the bone bodies in adjacent proximate position to promote healing therebetween. However, these techniques are not practical for certain joints such as joints formed between spinal vertebrae. Therefore, a significant number of stabilizing implants have been designed for joining together contiguous vertebrae.
One early technique for achieving arthrodesis between adjacent vertebrae across a joint or articulation is the well-known Cloward Technique for use in the human cervical spine. A solitary dowel of bone is tapped into place in a prepared circular bed that is smaller than the dowel of bone. The dowel acts as a wedge, distracting the surrounding soft tissues of the joint, and separating the bone bodies or vertebrae joined there along. The intervertebral disc substantially comprises the soft tissues of the joint. The dowel of bone is inserted, or wedged into place, providing its own stability by putting an annulus of the disc on stretch. Additionally, simple friction of the inserted dowel between adjacent vertebral bodies stabilizes axial dislocation. However, a second surgical procedure must be performed to extract or harvest the dowel of bone, substantially adding trauma to the procedure, increasing costs, as well as increasing the threat of infection to the patient. Alternatively, bank bone from human donors can be used, but bank bone is less osteogenic and may introduce infection, or even transmission of Acquired Immune Deficiency Syndrome (AIDS) or hepatitis. Furthermore, bone morphogenic protein, hydroxy apatite, or other bone stimulating material may be utilized. Additionally, there has been a need to ensure the implant remains axially secured which has lead to further developments.
A step forward from the Cloward Technique was provided by Bagby (U.S. Pat. No. 4,501,269) wherein a metal dowel uses the same principle. A perforated cylindrical hollow implant is inserted between prepared surfaces across a vertebral joint. The inserted implant immediately stabilizes the joint by spreading the bony surfaces apart in wedged opposition to surrounding tissue. This initial stabilization is more substantial because a metal dowel, unlike a bone dowel, will not be absorbed or fatigue in use. Over time, fusion occurs through and around the implant which is filled with bone fragments. Use of the metal dowel eliminates the need for a second operation to harvest a dowel of bone. Bone fragments to be inserted in the implant are retrieved during preparation of the circular beds in each vertebra. Furthermore, such a metal implant avoids the disadvantage of having to use a bone bank to obtain donor bone. The Bagby implant described in U.S. Pat. No. 4,501,269 has a smooth outer surface, interrupted only by numerous openings or fenestrations through which bone ingrowth and through growth can occur. Ends of the implant are substantially closed, with one end receiving an end cap such that bone fragments are contained therein. Bone morsels or bone grafts are typically harvested when preparing the circular bed in each vertebra, after which they are placed into the fenestrated metal cylindrical implant. The Bagby implant is then driven or tapped into place in a manner similar to the placement of Cloward's Bone Dowel, which was solely directed for use in the cervical spine. However, the original Bagby implant relies completely upon stretch of the annulus to stabilize the vertebrae during bone remodeling and fusion.
Improvements have also been made to “Cloward's Technique” wherein two dowel bone grafts are posteriorly inserted (Wiltberger's Technique) between adjacent lumbar vertebral bodies. Furthermore, threaded surfaces have been added to such bone grafts in order to keep the grafts in place (Otero-Vich German Application Number 3,505,567, published Jun. 5, 1986). More recently, a number of U.S. patents have proposed combining the threaded features from threaded bone grafts with a metal implant, resulting in rigid threaded implant structures for placement between adjacent spinal vertebrae.
One threaded metal fusion implant disclosed in Michelson (U.S. Pat. No. 5,015,247) provides a cylindrical fusion implant having an outer diameter sized larger than the space between adjacent vertebrae to be fused. Threads provided on the exterior of the member engage the vertebrae to axially secure the implant therebetween. The implant has a plurality of openings configured along the cylindrical surface to promote bone ingrowth. However, the threads per se of the implant do not function as a fastener to fix together the adjacent vertebral bodies. Instead, the implant functions as a wedge, imparting a distraction force across the disc which stabilizes the articulation formed therebetween by stretching the annulus of the disc. In fact, the threaded implant relies solely on the annulus to provide stabilization between the vertebrae, in direct response to wedge-induced distraction created therebetween. Distraction of the annulus stabilizes the two vertebrae, enabling ingrowth to later occur within the implant. Therefore, through-growth and fusion (arthrodesis) occur between the adjacent vertebrae subsequent thereto depending on the immobilizing potential of an intact healthy annulus which may or may not be present.
Several additional problems result from the provision of threads on a cylindrical fusion implant. One problem results in that threads take up additional space which makes implantation in areas having limited anatomical space very difficult, such as in the posterior approach in the lumbar spine. Additionally, the threads effectively make the wall thickness greater which further separates bone provided inside the implant with bone provided outside the implant, which can delay initial bone union.
For bone fusion to occur with any of the above devices, the invasion of new delicate blood vessels from the adjacent healthy bone is necessary for the creation of new living interconnecting bone. Where complete stabilization does not occur instantaneously upon implantation, motion can disrupt the ingrowth of delicate blood vessels. Disruption of the vessels then restricts or even prevents bone healing therebetween. The same problem occurs with any of the above-mentioned implant techniques, including the threaded techniques of Otero-Vich and Michelson. Even when the annulus is completely on stretch, the threads per se of these constructions do not function in the manner of conventional screws, extending through one object and into another. Namely, they do not function to fasten together adjacent bodies by coaction of the implant with each body. For example, the threads merely act as a series of ridges that engage with each adjacent bone body, while the implant body functions as a wedge. The implant distracts apart the vertebral bodies which stretches the annulus, and stabilizes the articulation as a consequence thereof, while the thread functions solely to prevent axial dislodgement. Furthermore, the presence of threads requires the implant to be screwed in place via a torquing process, instead of tapping the implant directly into position.
Hence, some recent designs have resulted in an implant that produces immediate fixation per se between bone bodies following insertion and independent of the annulus. Such designs show promise particularly for cases where the annulus structure is substantially or completely weakened or damaged at surgery. Where the annulus is damaged so significantly as to lose structural integrity, the wedge-effect of prior art threaded implants will not produce any distraction forces across the annulus. Also, when the implant is used to arthrodese and change angulation, a healthy annulus cannot be totally corralled to be placed on stretch. As a result, there is no form of stabilization or fastening between bone bodies sufficient to enable the occurrence of arthrodesis therebetween when the annulus is weakened or inadequate. Additionally, there exist additional shortcomings with such recent designs as discussed below.
One such design that produces immediate fixation is disclosed in Bagby (U.S. Pat. No. 5,709,683) as a bone joining implant having a spline or undercut portion that engages in assembly with each bone body to be joined. However, such design requires the preparation of bone beds that are engaged in interlocking relation with a bone bed engaging portion provided by such undercut portions.
Many of the previously described implants can be inserted between vertebrae while such vertebrae are distracted with a distraction tool. One such tool uses a threaded pin which is inserted laterally into each bone body, with such pins being rigidly secured therein. Such tool distracts the vertebrae by separating the pins and vertebrae which stretches the annulus. A drill is then used to drill out bone beds within the vertebrae, after which the implant is inserted therein. However, such procedure does not always impart sufficient distraction and takes time and space to implement.
Yet another group of implant designs provide distraction between adjacent vertebrae, including U.S. Pat. No. 5,665,122 to Kambin and U.S. Pat. No. 5,702,455 to Saggar. Kambin teaches an expandable intervertebral implant formed from several components that cooperate with an expansion screw to distract adjacent vertebral bodies by expanding two of the cage components relative to one another. However, such design is formed from several discrete components that are movably fastened together and which are susceptible of loosening and misadjusting within a patient. Saggar teaches a spine stabilizing prosthesis that is inserted within a cavity between vertebrae. Such design forms a jacking screw adjustment member that expands apart a pair of bearing members, each engaged with a respective vertebra. However, such design is illustrated in use as being inserted within a vertebral cavity that is formed by removal of a portion of a vertebra such as is formed by a corpectomy.
Therefore, there is a present need to provide an implant device that instantly fastens bone bodies together upon implantation, enhances arthrodesis by encouraging bony fusion adjacent the implant, and imparts distraction between adjacent bone bodies during insertion. There is also a need to provide such a device that facilitates staged stabilization leading to bone fusion, in a manner that is relatively simple, more reliable, less complicated, has fewer parts, and leads to quicker and more thorough bone fusion and remodeling therebetween. The final stage of bone fusion through and around the implant substantially eliminates any need for the implant to maintain the fusion, thus allowing the bone union to provide primary support therebetween.